Nitride semiconductors, for example, GaN, AlN, InN, and materials made from a mixed crystal thereof, have wide band gaps and have been used as high-output electronic devices, short-wavelength light-emitting devices, or the like. For example, GaN which is a nitride semiconductor has a band gap of 3.4 eV that is larger than the band gap of 1.1 eV of Si and the band gap of 1.4 eV of GaAs.
Examples of such high-output electronic devices include a field effect transistor (FET), in particular, a high electron mobility transistor (HEMT). Such a HEMT including a nitride semiconductor is used for high-output and high-efficiency amplifiers, high-power switching devices, or the like. Specifically, in a HEMT in which AlGaN is used for an electron supply layer and GaN is used for an electron transfer layer, piezoelectric polarization or the like occurs in AlGaN because of strain due to a lattice constant difference between AlGaN and GaN, and a high-concentration two-dimensional electron gas (2DEG) is generated. Consequently, such HEMT may operate at high voltages and be used for a high-voltage power device in a high-efficiency switching element, an electric car, or the like.
The 2DEG thus generated is usually present in a region under a gate even in a state in which a voltage is not applied to a gate electrode and the like. Therefore, a produced device becomes normally on. However, in general, it is desirable that a switching element for electric power and the like are normally off, that is, a current does not flow between a drain and a source in the case where a gate voltage is 0 V. Therefore, various structures and methods have been studied to ensure that the device becomes normally off. Meanwhile, a device having an insulating gate structure has been disclosed in which an insulating film is formed below a gate electrode to suppress a leakage current in a transistor such as a HEMT.
In order that a HEMT including a nitride semiconductor becomes normally off, the HEMT may have a structure in which a p-GaN cap layer is formed on an electron supply layer or a structure in which a gate recess is formed on an electron supply layer. However, crystal growth of the p-GaN cap layer is a difficult process and formation of the gate recess may be insufficient to become normally off. There is a method in which negative ions are implanted into a certain region of a semiconductor layer or a gate insulating film. However, nitride semiconductor layers such as an electron supply layer and an electron transfer layer may suffer significant damage by implantation of negative ions, which may cause, for example, the characteristics of a produced HEMT to be degraded, the uniformity of HEMTs to be degraded, and a yield may be degraded.
The following are reference documents:    [Document 1] Japanese Laid-open Patent Publication No. 2002-359256,    [Document 2] Japanese Laid-open Patent Publication No. 2011-14789 and    [Document 3] Japanese Laid-open Patent Publication No. 2010-199481.